1. Field of the Invention
The present invention relates to ignition compositions especially adapted to be used in electrically actuated initiation devices such as blasting caps, squibs, and the like.
2. Description of the Prior Art
Electric blasting caps comprise a cylindrical metal shell containing a train of powder charges. In some designs, the cap is actuated by applying current to a pair of leg wires whose ends are joined together inside the cap by a short length of high-resistance wire called the "bridgewire", which is embedded in the cap's ignition mixture. The heat produced in the bridgewire causes the ignition mixture to ignite. A similar technique may be employed for actuating squibs.
The degree of sensitivity of the ignition charge to impact, friction, electrostatic charges, and flame is always a matter of concern to manufacturers and handlers of electroexplosive devices. Except for the need for sensitivity to ignition by a hot bridgewire, low sensitivity under all other conditions naturally is the primary goal. Typical heat-sensitive exothermic-burning compositions which have been used as the ignition charge in bridgewire-fired ignition assemblies include the complex salt of lead nitrate with a lead salt of a nitrophenol, a 50/25/25 mixture of smokeless powder/potassium chlorate/dibasic lead salt of a nitrophenol, mercury fulminate, lead styphnate, lead mononitroresorcinate, tetryl/lead styphnate compositions, a 2/98 boron/red lead mixture, red lead/manganese boride, lead/selenium, etc.
An important consideration in the selection of the composition to be used as the ignition charge in an electric blasting cap is the environment to which the cap is to be exposed prior to being fired. Naturally, the composition must not decompose, or improper functioning, or non-functioning, may result. If the blasting cap is intended for use in oil wells that are deep and hot (for example, in systems designed to explosively perforate well casings and the wall of the oil well), consideration has to be given to the thermal stability of the ignition composition. As a rule, stability at temperatures up to about 350.degree. C. is desirable for such uses. For oil well caps, metal/oxide ignition compositions are preferred because they are more stable at higher temperatures. However, if oil is absorbed into the ignition composition in the liquid-disabled blasting cap described in the above cross-referenced co-pending application Ser. No. 469,954, too vigorous a reaction may occur between an oxidizer such as red lead (Pb.sub.3 O.sub.4) in a metal/oxide ignition mixture and the absorbed hydrocarbons at the temperature attained by the hydrocarbon in the oil well, or at the temperature of the heated bridgewire. Therefore, a need exists for bridgewire-initiatable ignition compositions which are stable at high temperatures, e.g., at about 350.degree. C. and above, and inert toward hot hydrocarbons.